The present invention relates to a gain control type amplifying apparatus, and more particularly to a gain control type amplifying apparatus which can precisely control an alternating current gain and a direct current level.
Generally, amplifying apparatuses is used for amplifying input signals, are classified into two types, that is, a constant gain type amplifying apparatus for amplifying an input signal at a given amplifying rate and a gain control type amplifying apparatus for freely varying the amplifying rate of the input signal.
In a conventional gain control type amplifier, a gain of the amplifier is controlled according to a resistance value of a variable resistance varied by the a user, which has a problem that the gain is not precisely controlled. Also, since a bias voltage of the conventional gain control type amplifier is fixed to a constant voltage, there is an additional problem that input signals less than the bias voltage are not amplified and are intercepted therein.
The above problems will be described below with reference to the attached drawing. FIG. 1 is a circuit diagram of a conventional gain control type amplifier. In FIG. 1, first and second transistors Q1 and Q2 are amplification a voltage divider composed of two resistors R3 and R4 and three diodes D1 to D3 connected in series between supply power V.sup.cc and ground GND, supplies a bias voltage to the base of second transistor Q2. A third transistor Q3 serves as an active resistance which controls the quantity of current on the current path of first and second transistors Q1 and Q2. Resistor R5 and variable resistor VR1 connected in series between supply power V.sup.cc and ground GND supply a control voltage for controlling the gain to the base of third transistor Q3.
As described above, the conventional gain control type amplifier has a disadvantage that input signals less than a bias voltage are intercepted because the bias voltage is fixed to a constant voltage and the gain control can not be also controlled precisely because a variable resistor in which its resistance value is variable by users, is used.